Although the name analytical chemistry originated from Boyle, its practice should be as old as chemical technology. The high development of ancient smelting and brewing technology is unimaginable, and there is no simple means of identification, analysis and control in the production process. The rise of alchemy and alchemy in the East and the West can be regarded as the precursors of analytical chemistry.
In 3000 BC, Egyptians knew about weighing technology. The earliest analytical instrument was the push-arm balance, which was recorded in the papyrus literature (BC 1300). The stone standard weights kept by the priests in Babylon (about 2600 BC) still exist today. However, the equal arm balance is used for analysis, and in the Middle Ages it was used to test gold in a baking pan (one of the fire detection methods).
In the 4th century BC, people knew to use touchstones to identify the fineness of gold.
In the 3rd century BC, Archimedes took advantage of the density difference between gold and silver when solving the purity problem of the gold crown of Herodias II, the ancient king of Syracuse, and was a pioneer of nondestructive analysis.
Around 60 AD, Pliny Sr painted the extract of gallnut on papyrus to detect the dopant iron (Ⅲ) in copper sulfate, which was the earliest organic reagent and test paper.
In 175 1 year, J. T. Erler von Brockhouse used the same method to detect the iron content in the bleeding residue (ashing). In 1663, Boyle reported the use of plant pigments as acid-base indicators. But the real ability analysis should be attributed to the Frenchman J.-L. Guy-Lussac.
In 1824, he published the determination of available chlorine in bleaching powder, using sulfonated indigo as indicator. Then he titrated plant ash with sulfuric acid and silver nitrate with sodium chloride. These three tasks represent redox titration, acid-base titration and precipitation titration respectively. Complexometric titration was invented by Justus von Liebig, who titrated cyanide ions with silver (Ⅰ). Another outstanding contributor to volumetric analysis is K.F. Moore of Germany, whose burette containing strong alkali solution is still in use today. He recommended oxalic acid as the reference material for alkali titration and ammonium ferrous sulfate (also known as molar salt) as the reference material for redox titration. The earliest microanalysis is chemical microscope, that is, observing the crystalline state, optical properties, particle size and spherical diameter of samples or reactants under the microscope.
/kloc-in the mid-7th century, R. Hook engaged in the research of microscopy, and published micro-micro in 1665. 1784, French pharmacist F.A.H Decca Lauzier distinguished potassium and sodium in the form of chloroplatinate with a microscope.
1747, German chemist A.S. Magraff confirmed with a microscope that sucrose and beet sugar are the same substance.
In 1756, platinum group metals are examined by microscope.
From 65438 to 0865, A. Hellwig wrote microscope in toxicology.
From 65438 to 0877, S.A. Boriji wrote "Analysis of Minerals and Rocks by Chemical/Microscopic Methods", in which gas reagents (such as hydrogen fluoride and chlorine gas), fluorosilicic acid and ammonium sulfide interacted with minerals and their flakes. T.H. Berens is not only engaged in the crystal inspection of inorganic substances, but also extends to organic crystals.
In 189 1, O. Laermann proposed thermal microscopy, that is, observing the changes of crystals when heated under a microscope. L Coffler and his wife designed two kinds of microscope heating tables, which are convenient for studying the identification of drugs and organic compounds. A thermal microscope only needs one crystal. It was later developed into an electron microscope with a resolution of 1 angstrom.
The earliest microanalyzer without microscope should be German J.W. Dobereiner. Engaged in wet micro-analysis, as well as blowing tube method and flame reaction, published a book "Micro-chemical Experimental Technology". The recognized founder of modern microanalysis is F Mickey. He designed and improved the micro-chemical balance to make its sensitivity meet the requirements of micro-chemical analysis, improved and put forward a new operation method, realized the determination of mg-level inorganic samples, and confirmed that the accuracy of nanogram-level samples was not lower than that of mg-level samples. The founder of organic micro-quantitative analysis is F. fritz pregl, who once separated a degradation product from bile, which was not enough for constant hydrocarbon analysis. On 1909, after listening to emich's lecture on micro-quantitative analysis and visiting his laboratory, he decided to change the constant combustion method to micro-method (a few milligrams of samples) and achieved success. 19 17 published the book Quantitative Analysis of Organic Trace, and 1923 won the Nobel Prize in Chemistry.
Constant operation needs to be improved if it is not suitable for microanalysis. For example, constant filtration is the quantitative transfer of sediment into a filter paper cone or filter crucible. If this method is used in micro-precipitation filtration, the substances attached to the beaker wall which was originally precipitated can no longer be ignored, so the method must be changed. Microfiltration is to suck out the mother liquor with a filter rod and leave all the sediment in the container. The container can be a 25 ml porcelain crucible or a weighing container. You can also wash off the sediment inside and then suck out the emulsion with a filter stick. This can not only avoid the loss of precipitation, but also simplify the operation procedure.
The behavior of inorganic compounds on filter paper has attracted people's attention in the19th century. German chemist F. F. Runge dropped the dye mixture on blotting paper in 1850 for separation. Previously, he used a piece of filter paper or cloth stained with starch and potassium iodide solution as a drop test of bleaching solution. He soaked the paper with ferric sulfate (ⅲ) and copper (ⅱ) solution, added yellow blood salt in the middle part, and added a second drop after inhaling one drop, thus obtaining the beautiful patterns he generated. 186 1, the capillary analysis of C.F. Schoenbein appeared. He immersed the filter paper strip in water containing several inorganic salts, and the water carried the "salt" upward along the paper strip, in which the water rose the highest, and other ions were divided into connected bands according to their "mobility". This is very similar to "paper chromatography". His students succeeded in the research of "separating organic compounds on filter paper", which can obviously and completely separate "organic dyes".
It is Fritz fritz pregl's contribution to detect inorganic matter and organic matter with filter paper or porcelain plate. This method is simple, selective and sensitive, and the drop test belongs to the category of microanalysis. Dropping Test and Chemistry with Specific, Selective and Sensitive Reactions are required reading books for analysts. 192 1 year later, Austrian F. Faigel systematically developed the drop test method.
In 1960s, H. Weiss put forward the ring furnace technology. Just put the microgram sample in the center of the filter paper, then wash it with solvent, and heat it at the outer edge of the filter paper to evaporate the solvent, thus it is divided into several concentric rings. If the ion is colorless, it can be sprayed with sensitive chromogenic agent or fluorescent agent. Detection and semi-quantitative results can be obtained. Chromatography, also called chromatography, is basically a separation method.
In 1906, Tswett, Russia added green leaf extract juice to the top of calcium carbonate precipitation column, and then carried out pure solvent leaching to separate chlorophyll. This study was published in the German journal Botany, so it failed to attract people's attention.
193 1 year, Germans R. Kuhn and E. Leder discovered this method again and showed its effectiveness. People traced it back to Tswett's research and earlier related research. For example, in 1850, J. T. Wei used soil columns for separation; 1893 L. Reed used kaolin column to separate inorganic and organic salts. Four years later, D.T. Day used bleached soil to separate oil.
Gas adsorption chromatography was proposed by P. Schuftan and A. Yuken in 1930s. In the 1940s, German Y. Hesse used gas adsorption to separate volatile organic acids. An Englishman, e grukov, also used the same principle to separate helium and neon in the air in 1946, and made a gas chromatograph in 195 1 year (see gas chromatography). The success of the first modern gas chromatograph should be attributed to E. Kramer.
According to the principle of liquid-liquid distribution, the gas-phase partition chromatography was proposed by A. J.P Martin and R. L.M Singer in England in 194 1 year. Because of the importance of this work, they won the 1952 Nobel Prize in chemistry. M. J.E Golay proposed that the use of long capillary columns is another innovation.
Chromatography-mass spectrometry (GC-MS) is one of the most effective analytical methods. Complex organic mixtures can be separated and identified within a few hours by transferring the extracted liquid obtained by chromatography into the mass spectrometer.
Liquid chromatography includes liquid-liquid chromatography and liquid-solid chromatography. The first state of the latter two names represents the mobile phase and the second state represents the stationary phase. At atmospheric pressure, the flow rate of liquid chromatography is too low, so the pressure must be increased. The pioneering work in this field is the separation of amino acids by high pressure liquid chromatography in 1960 by P.B. Hamilton.
J.C. Giddings pointed out in 1963 that the column efficiency of liquid chromatography should catch up with that of gas chromatography. The filler particles of the former are much smaller than that of the latter, so it needs a lot of pressure and the pump used should be pulse-free.
In 1966, R. Gentford and T. H. Gao Made invented this pulse-free pump.
In 1969, J.J. kirkland improved the filler to make it have a specified surface porosity, and then bonded the stationary phase (such as n-hexadecyl) to the carrier to make it resistant to heat and solvent decomposition. The carrier can be silicon dioxide bonded by Si-O-C or Si-C bonds. Thin-layer chromatography uses thin-layer silica gel instead of filter paper for chromatographic separation. Because silica gel particles are uniform and fine, the speed and degree of separation are generally better than that of paper chromatography, and it is as effective as paper chromatography in separating inorganic substances and organic substances.
1889, Dutch biologist m.w. Bayer Jinker dropped a drop of mixed solution of hydrochloric acid and sulfuric acid in the middle of the thin layer of animal glue. Hydrochloric acid further diffuses to form another ring outside the sulfuric acid ring. Silver nitrate and barium chloride were used to successively show the existence of these two rings.
Nine years later, H.P. Vi Hysmans used the same method to prove that amylase in malt actually contains two enzymes.
Until 1956, E. Shtal of the Federal Republic of Germany improved the coating method and operation, and adopted measures such as fine particle (0.5 ~ 5 micron) silica gel, which made the method widely used. Quantitative thin-layer chromatography began with J.G. Kirschner et al. (1954). They determined biphenyl in citrus and its processed products for the first time (see TLC). The Greek philosopher Ty olaf Stuss recorded the influence of various rocks, minerals and other substances on heat. Le Chatelet of France and Robert Austin of England are the founders of differential thermal analysis.
In 1960s, precise differential thermal analyzer and differential scanning calorimetry proposed by M.J. O 'Neill appeared, which can determine the purity and other parameters of compounds, such as melting point, glass transition temperature, polymerization temperature, thermal degradation temperature and oxidation temperature (see thermal analysis).
At the beginning of the 20th century, thermogravimetry was proposed to study the weight changes of materials such as steel and precipitates when heated. Bendo Kotaro invented the first thermal balance instrument, which was originally used only to solve metallurgical problems. Use it to analyze when pushing C. Duval. He studied the thermal behavior of 1000 kinds of sediments. For example, calcium oxalate can be burned into calcium oxide at high temperature or calcium carbonate at about 550℃. As a weighing form, the latter is better, because it not only saves energy when burning, but also has a large conversion coefficient (so the error is small), which prevents calcium oxide from absorbing moisture when weighing.
In electrolysis, copper (Ⅱ) is reduced at the cathode, precipitated as a simple substance (zero valence), and then weighed, which should be classified as gravimetric method. At this time, electrons can be considered as precipitants. There is also lead (II) in anodic oxidation, which is attached to the anode in the form of lead dioxide. The former law was independently put forward by German C Luko and American j·w· Gibbs in the 65438+60s. /kloc-at the beginning of the 9th century, only oxalic acid, its ammonium salt and ammonium succinate were used as organic reagents for inorganic gravimetric analysis. The former is used for the separation of calcium and magnesium and the determination of calcium. The latter is used to precipitate trivalent iron and separate it from divalent metal ions.
In 1885, M.A. Ilinski and G.von Knorre proposed that 1- nitroso -2- naphthol is the precipitant of cobalt in the presence of nickel and the first chelating agent. As for the determination of anions, at the beginning of the 20th century, W. Miller proposed to use 4,4-benzidine as the precipitant of sulfate.
In 1950, China Liang Shuquan et al. used organic reagents for gravimetric analysis to determine tungstate.
In 1950, M. Bush introduced 4,5-dihydro-1, 4- diphenyl-3,5-phenylimino-1, 2,4-triazene (nitrate reagent for short) as nitrate precipitant. 1975 becomes a good precipitant for perrhenate.
In 1950, dimethylglyoxime was synthesized by л A. Chu Gaeff, and it was observed that it formed a red precipitate with nickel (Ⅱ). Two years later, O.E. Brock of the Federal Republic of Germany applied dimethyl oxime reagent to the determination of nickel in steel. Since then, sensitive and highly selective new organic reagents have emerged. China Ceng Yun et al. synthesized 3-(2- arsanilino-phenylazo)-6- (2,6-dibromo -4- chlorophenylazo) -4,5-dihydroxy-2,7-naphthalene disulfonic acid. When using this reagent, the molar absorption coefficient of rare earth elements can be as high as 0.98 ~ 1.2. This is an analytical method based on the selective absorption of light by the measured substance molecules. Include colorimetric analysis and ultraviolet-visible spectrophotometry. Measure the absorption degree of the solution to monochromatic light with different wavelengths, and plot with wavelength as abscissa and absorbance as ordinate to get the absorption spectrum. According to all the special absorption spectra of various substances, qualitative analysis and quantitative analysis can be carried out.
Colorimetry takes the sunlight as the light source, and visually compares the shades of colors. The earliest record is that 1838 w. a. lamadius determined iron and nickel in cobalt ore in a glass measuring cylinder, and compared the standard reference solution with the sample solution.
In 1846, A. Jacquelin proposed to determine copper according to the blue color of copper ammonia solution. Then the thiocyanate method of T.J. Heropas was used to determine iron (1852); Determination of ammonia by Nessler method: determination of nitrate by phenol disulfonic acid method (1864); determination of titanium by hydrogen peroxide method (1870); determination of hydrogen sulfide by methylene blue method (1883); determination of silicon dioxide by phosphosilicic acid method (1898). The spectrophotometer adopts monochromatic light and photomultiplier tube, and the wavelength range is 220 ~ 1000 nm, which is wider than the visible range (400 ~ 700 nm).
When the suspension is illuminated by light, when the line of sight is at right angles to the light from above, it is called fog comparison method; If the line of sight and light are in a straight line, it is called turbidimetry.
1In the 1950s, G.J. mulder used visual method to measure the brightness of silver chloride suspension in the upper liquid. Subsequently, J.-S. Starr used a standard suspension as a reference.
1894, T.W. Richards of America designed the first haze meter. The specific fog method was originally used to observe whether the concentrations of chlorine (or bromine) ions and silver ions in mother liquor reached the equivalent level in atomic weight determination. Subsequently, this method is used for quantitative determination, with high sensitivity, which can determine 3 micrograms of phosphorus in a liter of water or 10 micrograms of acetone in a liter of water. Infrared spectroscopy is a powerful means for organic chemists to identify unknown compounds. Infrared spectroscopy was applied to the study of gasoline knock in the 1920s, and then it was used to identify the unknowns and impurities in natural and synthetic rubber and other organic compounds. In 1970s, based on the rapid development of computer, Fourier transform infrared spectroscopy (FTIR) experimental technology entered the laboratory of modern chemists and became an important tool for structural analysis. Far infrared spectrum (200 ~ 10 cm) and microwave spectrum (10 ~ 0. 1 cm) are spectral methods to study molecular rotation.
Raman spectroscopy is another method to study molecular vibration. The early Raman spectrum signal was too weak to be used, and it was not until laser was used as a monochromatic light source that its application in analytical chemistry was promoted. Raman spectroscopy has developed into Fourier transform Raman spectroscopy analysis technology, focusing micro Raman spectroscopy analysis technology, surface enhanced Raman effect analysis technology and so on, which plays an important role in biomedical analysis, cultural relics analysis, gem identification, mineral analysis and other fields. 1672, Newton used a prism to divide sunlight into seven colors in a dark room, which was the ancestor of atomic emission spectrometry.
1800, f.w. herschel discovered infrared rays. The following year, J.W. Ritter discovered the ultraviolet region by using the reduction phenomenon of silver chloride. The following year, W.H. wollaston observed the dark line of the solar spectrum.
18 15 years, J.von Fraunhofer named the dark line as the Fraunhofer line after research. In the literature, the sodium line is called D line, which is also stipulated by Flawn Hof. Bunsen burner gas lamp was invented by R.W. Benson, whose flame is almost transparent and does not emit light, which is convenient for spectral study.
1859, Bunsen and his colleague physicist G.R. Kirchhoff studied the characteristic emission and absorption spectra of the elements in the flame, and pointed out that the Fraunhofer line in the solar spectrum is an atomic absorption line, because there are various elements in the atmosphere of the sun. The instruments they use already have the elements of modern spectroscope. They are the founders of emission spectrometry. Chemical analysis includes titration analysis and weighing analysis, and the composition and relative content of substances are determined according to their chemical properties.
spectroscopy
mass spectrometer/spectrograph/spectroscope
Spectrophotometry and colorimetry
Chromatography and electrophoresis
crystallography
Microscopic observation
electrochemical analysis
Classical analysis
Although most contemporary analytical methods are instrumental analysis, some instruments are originally designed to simplify the inconvenience of classical methods, and their basic principles still come from classical analysis. In addition, sample preparation and other pretreatment still need the assistance of classical analysis methods. The following are some classic analysis methods:
titration
gravimetric analysis
Inorganic qualitative analytical instruments: Contemporary analytical chemistry focuses on instrumental analysis. There are several commonly used analytical instruments, including atomic and molecular spectrometers, electrochemical analytical instruments, nuclear magnetic resonance, X-ray and mass spectrometry. Analytical chemistry methods other than instrumental analysis are collectively called classical analytical chemistry.
Analytical chemistry is an important branch of chemistry, which mainly studies which elements or groups are in substances (qualitative analysis); What is the quantity or purity of each component (quantitative analysis); How atoms are connected into molecules, how they are arranged in space and so on.
Instrumental analysis: it is to determine the composition and relative content of a substance according to its physical or physical and chemical properties. Instrumental analysis can be divided into four categories according to the different determination methods and principles: electrochemical analysis, optical analysis, chromatographic analysis and other analysis methods. As shown on the right.
Main analytical instruments:
Atomic absorption spectrometry
atomic fluorescence spectrometry
Alpha particle x-ray spectrometer (APXS)
Capillary electrophoresis analyzer
Chromatography (chromatography)
colorimetry
cyclic voltammetry
Differential scanning calorimetry (DSC)
electron paramagnetic resonance
electron spin resonance
Ellipsometer (ellipsometer)
Field flow fractionation (FFF)
Fourier transform infrared spectroscopy
gas chromatography
Gas chromatography-mass spectrometry
High performance liquid chromatography (HPLC)
Ion microprobe
inductively coupled plasma
Instrument mass fractionation (IMF)
ion selective electrode
Laser induced breakdown spectroscopy (LIBS)
mass spectrometric analysis
Mossbauer spectral system (Mossbauer spectral copy)
nuclear magnetic resonance
Particle induced x-ray emission spectroscopy (PIXE).
Pyrolysis-gas chromatography-mass spectrometry
Raman spectrum (Raman spectrum)
refractive index
* * * Resonance enhanced multiphoton ionization (REMPI)
Scanning transmission x-ray microscope (STXM)
thin-layer chromatography
Transmission electron microscope
X-ray fluorescence spectrometer
X-ray microscope (XRM) chemical analysis and instrumental analysis.
Any method that mainly uses chemical principles for analysis is called chemical analysis; The method of analysis mainly using physical principles is called instrumental analysis. Of course, it is difficult to draw a clear line between the two, and there are ways in the middle.
Instruments generally refer to large-scale instruments, such as nuclear magnetic resonance (see nuclear magnetic resonance spectrum), X-ray fluorescence spectrometer (see X-ray fluorescence spectrometer), X-ray diffractometer, mass spectrometer (see mass spectrometry), electron spectrometer, etc. Atomic emission spectrometry and atomic absorption spectrometry are basically wet pretreatment and then determined in corresponding instruments, which can be regarded as a method between them, or as a combination of chemical methods and instrumental methods. You can't think that the use of instruments is instrumental analysis. For example, gravimetric analysis begins with weighing the sample with a balance, and the last step is weighing the precipitate with a balance.
Balance is a physical instrument, and weighing is a physical process, but gravimetric analysis is recognized as a typical chemical analysis method, because gravimetric analysis mainly relies on the interaction between ions to be measured and precipitants to quantitatively precipitate precipitates. As for the word classical method, it refers to gravimetric analysis and volumetric analysis. Its scope is far narrower than chemical methods. Therefore, the classical method is only a part of chemical analysis, not the whole. It is roughly divided into inorganic analysis and organic analysis.
The analysis of inorganic substances in natural products and industrial products, such as rocks, minerals, ceramics, steel, alloys, inorganic acids, caustic soda, etc., belongs to inorganic analysis; The analysis of petroleum, dyes, plastics, food, synthetic drugs and Chinese herbal medicines belongs to organic analysis. In short, the analysis of hydrocarbons and their derivatives belongs to organic analysis, while the analysis other than the above substances belongs to inorganic analysis. But sometimes inorganic substances are mixed with some organic substances, and organic substances also contain inorganic substances. For example, river water and seawater contain organic matter, some manganese ores contain organic matter, coal contains ash, petroleum contains metal in complex form, and paper contains inorganic filler. This kind of article uses both inorganic analysis and organic analysis.
There are other methods that are equally effective for inorganic substances and organic substances, such as gas chromatography. Carbon monoxide, carbon dioxide, hydrogen, nitrogen, oxygen, methane, ethylene and water vapor in the sample can be separated one by one or in groups under selected conditions. The same is true of Hosatte gas analyzer, but the separation principle is different.
Trace analysis means that the sample contains very little. The content in general samples is the main component, and the content is the secondary component. E.b. Sandel thinks that the content of 1% ~ 0.0 1% is the secondary component. Some people think that 10% ~ 0.0 1% is a secondary component. The content below 0.0 1% is called trace. The trend of trace analysis tends to determine lower and lower content, so ultra-trace analysis appears, that is, the content is close to or lower than the lower limit of general trace. This name is only qualitative. For quantitative or more specific names, please refer to the following provisions:
Trace 10 ~ 10 μ g/g
Trace 10 ~ 10 μ g/g
Nanometer trace 10 ~ 10 μ g/g
Sand trail 10 ~ 10
Microgram/gram microanalysis has another meaning, that is, trace elements (such as
② The precision and accuracy are the highest;
③ The sensitivity is the highest, which can verify and determine trace or trace components;
④ The determination range is the widest, and both large and small amounts can be determined;
⑤ The types and types of elements that can be determined are the most;
⑥ The method is simple, that is, it is the easiest to operate without high skill;
⑦ It is economical and practical, that is, it requires less expenses and greater benefits. However, it is impossible to combine all the advantages in one method. For example, in gravimetric analysis, if the accuracy is to be improved, it is necessary to extend the analysis time (for example, to purify the precipitate by heavy precipitation). The chemical method is the most time-consuming because it needs to be accurate to1100,000. The analysis method should be simple and not only suitable for field work (such as geological survey, chemical exploration, environmental monitoring, soil detection, etc.). ), also used for routine indoor analysis.
Because under the premise of not losing the required accuracy and precision, the method is simple and has few steps, which means saving time, manpower and cost. For example, when a gold shop buys gold ornaments, it draws a line (scientific name is stripe) on the touchstone board, and then identifies the fineness of gold from the color of the stripe. This fringe method is still used for mineral identification.
Of course, this method is not as accurate as fire assay or atomic absorption spectrometry, but it can achieve the purpose of identifying gold wares. For another example, the urine sugar content of diabetic patients can be detected by special enzyme-containing test paper, and the sugar content can be estimated from the color change of the test paper. The method is simple and can be used by patients themselves. On the other hand, although atomic absorption spectrometry can indirectly determine the sugar content in urine samples, it is not adopted because it is uneconomical. Although there are many sensitive, selective (even specific) methods, if the concentration of the elements to be detected is close to or lower than the lower limit of the method, enrichment is still inevitable. There are many enrichment methods, such as sublimation, volatilization, distillation, foam flotation (see trace enrichment), adsorption (using molecular sieve, activated carbon, etc. ), chromatography, * * precipitation, * * crystallization, amalgam, selective dissolution, solvent extraction, ion exchange and so on.
Before detection or determination, it is often necessary to separate the substances to be detected (or detected) from the interfering substances. Important separation methods include distillation, solvent extraction, ion exchange, electrodialysis, precipitation, electrophoresis and so on. They are basically the same as the concentration method. Concentration can be considered as a separation method to increase concentration.
Although concealment (see concealment and non-concealment) is not separation, its function is to make ions lose their normal properties, that is, to exist in the reaction system in another form. The purpose of separation in analytical chemistry is to make interfering ions stop interfering, so in a broad sense, masking and its opposite effects should be included in the category of separation. Hiding and exposure have been used in analytical chemistry for a long time. Gravimetric analysis, spectrophotometry and polarography all have applications, especially in drop test and complexometric titration. The most important requirement of sampling is representativeness, that is, the sample to be analyzed must be representative of all. It is not a problem to sample homogeneous or miscible substances, and both gaseous and liquid samples belong to this category. Uneven solid materials, such as ore and coal, should be sampled according to the prescribed procedures. Otherwise, the analysis results can't represent the original materials, wasting manpower and material resources. Field ore sampling is mostly carried out by geologists. In the laboratory, the analyst smashed the obtained large samples and reduced them to small samples according to certain procedures. However, the pure samples synthesized by organic element combustion method do not have this problem.
Sample melting is the second step. Dissolution includes dissolution and melting, also known as decomposition. Some samples are soluble in water, acid or mixed acid, alkali and organic solvent. If the above method cannot be dissolved, it can be melted with flux. Fluxes can be divided into alkaline (such as sodium carbonate), acidic (such as potassium bisulfate), oxidizing (such as sodium peroxide) and reducing (such as sodium thiosulfate). If the components to be analyzed are volatile or the melting temperature is high, which seriously corrodes the crucible, sintering can be used instead, that is, the surface of the particles is partially melted. Smith method is an example of mixing and sintering ammonium chloride and calcium carbonate (1: 8 ~ 12) with silicate rocks to determine the alkali metals in them. Organic compounds and biological samples can be incinerated by dry or wet methods. Dry ashing refers to heating to carbonization and gradual combustion under the condition of sufficient oxygen, or oxidation with atomic oxygen at a lower temperature (low temperature ashing). Wet ashing uses oxidizing acids (such as nitric acid, perchloric acid and concentrated sulfuric acid) to oxidize samples. Dry method and wet method have their own advantages and disadvantages, depending on the sample.